Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing “smart windows” with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective because the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Another limitation of electrochromic devices is that many employ organic dyes that have limited stability to ultraviolet light; absorb light rather than reflect it and therefore only partially reduce solar thermal burdens; and do not offer privacy or complete opacity. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications, e.g., energy efficient dimmers for displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte twice. There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
In prior art attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of metal electrodeposition at the counter electrode). Such teachings imply that the application of reversible metal electrodeposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside.
In addition to the desired electrodeposition of metals, competing and deleterious side-reactions may also occur. If aqueous electrolytes are used with the device, then the oxidation and reduction of water to produce oxygen gas and hydrogen gas, respectively, may occur. These reactions decrease the current efficiency for metal electrodeposition. Importantly, reversible mirror devices are volume-constrained and may rupture if even small amounts of gas are generated. Other unwanted and deleterious side-reactions include formation of metal complexes with hydroxide and oxide ligands, and rapid oxidation of electrodeposited metal.
Some of the problems associated with the use of aqueous electrolytes may be overcome if organic solvents are used instead of water. Reversible mirror devices that employ conventional organic solvents have been described in, for example, U.S. Pat. No. 5,923,456 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror,” which issued on Jul. 13, 1999; U.S. Pat. No. 6,111,685 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror (REM) With Improved Electrolyte Solution,” which issued Aug. 29, 2000; U.S. Pat. No. 6,166,847 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror for Modulation of Reflected Radiation,” which issued Dec. 26, 2000; and U.S. Pat. No. 6,400,491 to D. M. Tench et al. entitled “Fast-Switching Reversible Electrochemical Mirror (REM),” which issued Jun. 4, 2002, all incorporated by reference herein. Organic solvents, however, present their own set of problems for reversible mirrors. These problems may include low solubility of charge carriers in organic solvents, poor conductivity, poor solubility of metal ions, low boiling points, toxicity, flammability, low electrochemical stability, low photostability, and poor seal tolerance. Some organic solvents are reduced (and generate hydrogen gas) more easily than the metal ions are electrodeposited. Additionally, organic solvents that are polar enough to support electrochemistry may include functional groups such as ketone groups and ester groups that may chemically react with metals and interfere with electrodeposition.
Another reversible mirror employs a permanent thin metal film that becomes transparent upon exposure to hydrogen gas (see, for example, U.S. Pat. No. 6,535,323 to M. T. Johnson et al. entitled “Light-switching device,” which issued Mar. 18, 2003; U.S. Pat. No. 5,905,590 to P. Van Der Sluis et al. entitled “Optical switching device comprising switchable hydrides,” which issued May 18, 1999; U.S. Patent Application 20020044717 to T. J. Richardson entitled “Electrochromic materials, devices and process of making,” which was published Apr. 18, 2002; and T. J. Richardson et al., “Switchable mirrors based on nickel-magnesium films,” Applied Physics Letters, 2001, 78, 3047-3049, all incorporated by reference herein). These devices operate either by addition and removal of hydrogen gas, which requires gas handling capabilities and the manipulation of highly flammable hydrogen gas, or by electrochemical production of hydrogen gas from highly caustic aqueous solutions.
A reversible mirror that overcomes the disadvantages of known reversible mirrors remains highly desirable.
Therefore, it is an object of the present invention is to provide a reversible mirror that overcomes the disadvantages of known reversible mirrors.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.